Schwartzmann was beaten 6-3 6-7 (4-7) 6-3 6-3 by Nadal on Rod Laver Arena on Sunday as he set up a quarter-final meeting with Marin Cilic.
The No 24 seed made life tough for Nadal and had him worried when he took the first set.
But Nadal roared back to secure a win in 3hr 51mins.
On what it’s like to face the Spaniard, Schwartzmann said: “I think he’s really aggressive every time. He can play four hours aggressive.
“He can play four hours… He is really, really intensive. It is not easy for me.
“For example, on the court four hours, trying to run every point, trying to do winners every point, it is not easy to be focused four hours against him.
“He can do that easy. And I think that is the best thing from him.”
Nadal, meanwhile, paid tribute to his beaten opponent.
“It was a great battle. He is a good friend of mine,” he said.
“He is a great player, in all aspects, and I feel that for a lot of moments he was serving well.
“I was able to keep fighting until the end and it is the first big match I have played in 2018. It probably helps my confidence, knowing I can resist almost four hours on court.”